Oscillator circuit



Feb. 11, 1958 F. v. TOPPING OSCILLATOR CIRCUIT Filed June 26. 1956 I.lnventor FREDERICK .V. TOPPING #flf. Attys Unite 1 OSCILLATOR CIRCUITApplication June 26, 1956, Serial N 593,966

1 Claim. Cl. 250-3 This invention relates to oscillators wherein avacuum tube is coupled to a resonator (usually .a crystal) and toamplifying systems coupled therewith, wherein the magnitude of theoscillation is limited by the voltage between the control grid and thecathode of the tube.

Two common types of oscillators wherein the oscillations are limited insuch a way are the Pierce and Miller types. Oscillators of this typeutilize a vacuum tube in combination with a resonator, generally acrystal and the control grid of such tube is connected to ground througha grid leak resistor.

In any such oscillator means must be provided whereby the amplitude ofthe oscillations may be limited. In prior oscillators such limiting isprovided by the grid bias developed across the grid leak resistor due tothe conduction between control grid and cathode when the control gridbecomes or approaches a' positive potential with respect to the cathode.The effect of such conduction has, however, been to introduce into theresonator circuit the rectification phenomena occurring between thecontrol grid and cathode. Such phenomena take the form of unwanted andunpredictable reactance or resistance effects which when introduced intothe resonator circuit result in undesirable variations in the outputfrequency of the resonator.

This invention therefore provides circuitry for oscillators of the typedefined in paragraph two above wherein the limiting effect is obtainedby providing a crystal diode rectifier between the control grid andground which is independent of the tube grid and cathode. The crystaldiode rectifier is oriented to conduct when the control grid is positivewith respect to ground. In addition the tube cathode is connected toground through a cathode bias resistor. the conduction of the diodewhenever the control grid tends to become positive. Since the controlgrid and cathode of the tube are not used for rectification, noconduction takes place therebetween, hence the control grid-cathoderesistance and reactance effects are not introduced into the oscillatorcircuit. Although there is a tendency in tubes for conduction to takeplace when the control grid potential is almost at, but less than thecathode potential, due to contact potential effects; such conduction isavoided in the novel circuit by the positive biasing of the cathode byits resistor which maintains a potential difference between cathode andgrid, greater than the potential at which conduction could be initiated.The result is a more accurate oscillator than it has previously beenpossible to develop without complex circuitry.

Such an oscillator is coupled to an amplifier system so that a suggestedarrangement would be: the oscillator stage, separated from the amplifierstage by a buffer stage, the buffer stage preventing load effects frombeing superimposed upon the oscillator circuit.

It is common to provide the amplifier with a tuned output circuit, thetuning being to the frequency of the oscillator. Thus if a crystal of adifferent frequency is sub- The oscillations are therefore limited bySttes Patent stituted for the first crystal, retuning of the tunedcircuit is required.

In an oscillator this retuning requires the inconvenience of addedadjustments and the expense of added parts.

In accord with the present invention the output of the system isobtained from the secondary winding of an untuned transformer whoseprimary winding forms the plate load for the tube of the amplifierstage. Since the output is untuned there is thus no needfor tuning whena crystal of different frequency is substituted and therefore no tuningapparatus is required.

The connection above described is used with, class A amplification.

In a drawing which illustrates a preferred embodiment of sufficientlyhigh value to avoid appreciable resistive loading of the crystal. v

The cathode is connected to ground through cathode bias resistor 21 andthrough radio frequency grounding condenser 22. a

One side of a resonator (preferably a-crystal) is connected to the platethrough blocking condenser 15. The other side of the crystal or otherresonator is connected to the control grid of tube A.

In parallel with the crystal is a variable condenser 12. Condenser 15and one side of the crystal 11 are grounded through condenser 13 whilethe other side of the crystal is grounded through condenser 14. Variablecondenser 12 in conjunction with condensers 13 and 14 form the loadcapacity for the crystal. Condensers 12 to 14 constitute the resonatorload capacity.

The control grid of tube A is provided with a grid leak resistor 19connected to ground. One of the novel features of this invention is theprovision of a crystal diode rectifier in parallel with the grid leakresistor with the polarity arranged so that the rectifier conducts whenthe grid is positive with respect to ground. The addition of the crystaldiode connected in this way has been found to add to the accuracy and/or stability of the oscillator.

It will be noted that the magnitude of the oscillations is limited bythe fact that the crystal diode never allows the control grid to becomemore positive than ground potential. Since the cathode of the tube isbiased positively by the cathode bias resistor, conduction between gridand cathode is avoided. Thus by the use of the crystal diode nogrid-cathode resistance or reactance efiects are introduced into theoscillator circuit.

The screen grid of the pentode is R. F. grounded by condenser 24. Thepositive source is connected to ground through a potentiometer resistor25 which by its adjustable contact sets the potential of the screengrid. The adjustable contact is connected to the screen grid by seriesresistor 23 which is used to give the required g characteristics to thetube and also to assist by-pass condenser 24 to return the radiofrequency again to the common ground.

The butler stage B is operated as a class A amplifier, it will be seenthat the pentode therein receives a sinusoidal output of the oscillatorstage at its control grid from the side of the crystal connected to theplate of the oscillator tube A. A sinusoidal output may also be derivedfrom the side of the crystal connected to the control grid.

It is also possible to derive a pulsating output signal from theoscillatortubeby conventional methods but if this is done elements (notshown) must be provided. in buffer. or amplifier stages to convert suchpulsating signals to sinusoidal form.

The plate of the butter pentode is connected to the positive potentialsource 43 by plate resistance 31. The cathode is grounded throughcathode biasresistor 28 and also through R. F. by-pass condenser 29.

The control grid is grounded through. grid. load resistor 27 which inconjunction with the oscillator stage plate resistor 18 forms part ofthe resistiveloading for the crystal 11.

Voltage for the screen grid of the buffer stage B is supplied throughresistor 34 andthescreen grid is R. F. by-passed by condenser 30.

Referring now to the amplifier stage Cit, is seen, that the pentodetherein receivestheoutput of the oscillator stage at its control gridwhich receives the output from the plate of the butter stageB. The plateof the amplifier C is connected to the positive source-throughtheprimary winding of an untuned transformer 37 and a de-couplingresistor 36. The plate is R. F. by-passedby condenser, 40 and thede-coupling resistor is located between the voltage source and theprimary winding.

A filter condenser 35 connecting the positiveline to ground betweensource and resistor. 26 assists in;th e, decoupling operation.

The cathode of the amplifier stage is connected; to ground throughcathode bias resistor 40 andR, F. bypass condenser 39. The control gridis groundedihrough grid load resistor 38.

The oscillations soproduced are supplied through the buffer stage to theamplifier from which the output is obtained at the secondary oftransformer 37. Since this output is untuned there is no tuning requiredwhen the crystal frequency is changed by the substitution of a furthercrystal. Class A amplification is used in the amplifier stage.

It will be obvious that providing a crystal diode in parallel with thegrid leak resistor of the oscillator stage in accord with theinventionmay not only be performed with a Pierce oscillator but alsowith a. Miller oscillator and in fact with any oscillator system whereinthe magnitude ofthe oscillations,is.lirnited by-the voltage betweencontrol grid and. cathode.

It should be noted that the crystal resonator used with the oscillatorstage may be replaced by. any other, suitable resonator.

What I claim as myinvention is:

In an oscillator including a resonator having a pair of terminalscoupled to a vacuum tube of the class wherein the magnitude of theoscillations is limited by the voltage betweericontrol grid and cathode:a cathode bias resistor, a grid leak resistor connecting ai controlgrid'to ground, a crystal diode rectifier in parallel With saidresistor, with its polarity, arranged toconduct when the control grid ispositive with respectto the ground, and a condenser connected betweeneach of said resonator terminals and ground.

References Cited in the file of this patent UNITED STATES PATENTS OTHER.REFERE CES Page. 504; (:Fig, 23); of? Radio Engineersi Handbook byTerman; 1943 edition.

